A conventional pipetting process in an automatic analyzer typically involves, for example, removal of a given volume of reagent from a primary vessel, for example, a reagent container, and delivery thereof to a secondary vessel, for example, a reaction vessel. A conveying device conveys the pipetting needle from one target position to the next. In each target position the automatically controlled metering device removes or delivers the desired volume of liquid.
In the three-dimensional rectangular coordinate system of the conveying device, the primary and secondary vessels are disposed in containers on carrier plates extending parallel to the XY plane of the coordinate system. When the conveying device moves the pipetting needles to a target position, it first moves the needle in a plane parallel to the XY plane until it is above the target vessel, after which it moves the needle downwards into the correct position for removing liquid from a primary vessel or delivering liquid to a secondary vessel.
If the conveying device is to convey the pipetting needle to exactly the right target position in the XY plane, i.e. exactly above a primary or secondary vessel, the following conditions must be fulfilled: the device controlling the conveying device must receive an input signal containing the X and Y coordinates of the target position in order to actuate the conveying device accordingly; the primary and secondary vessel must be at exactly the right position; the pipetting needle must be straight, i.e. must not have any deformation; and the conveying device must be adjusted in each conveying direction, i.e. in each conveying direction a position must be defined and serve as a reference position.
In order to adjust the conveying device of the pipetting needle, in previously-known pipetting devices of the aforementioned kind, the reference position of one component of the device for conveying the pipetting needle is manually adjusted for each direction of conveyance. The known adjustment process is based on adjusting the position of mechanical parts of the conveying device in relation to reference points having positions (called zero positions) defined e.g. by light barriers. One such zero position must be defined for each conveying direction. The zero positions must also be adjusted until the distance of the light barrier from the primary vessels has a defined value. When the device is used, the aforementioned mechanical devices are automatically moved to their zero positions whenever the device is switched on.
This known method of adjustment has the following disadvantages:
(1) Relatively considerable labor and time is required for basic adjustment of the device conveying the pipetting needle when the device is set up.
(2) The adjustment process cannot take account of possible deformations of the pipetting needle during its working life, i.e. even if the reference position of the aforementioned components of the conveying device is relatively accurately adjusted, it may happen that a slightly bent pipetting needle misses its target.
(3) The accuracy of adjustment of the zero positions decreases with ageing of the light sources of the light barriers. The brightness and width of the light beam vary with the age of the light source. This results in a corresponding change in the zero positions. To take account of this and to ensure reliable operation of the pipetting devices, the zero positions have to be readjusted by a service engineer after some years. This readjustment also requires a relatively large expense of labor and time.
(4) The accuracy attainable by the known adjustment of the reference position is limited by the fact that a number of mechanical components are inserted between the pipetting needle and the element whose reference position is adjusted. The accuracy with which the needle can be positioned by the conveying device is therefore limited by the sum of the conventional deviations in the dimensions of the aforementioned components and by the spaces between the components.
The known adjustment method is therefore unsuitable for pipetting devices where the needle-conveying device has to be very accurately adjusted and manual adjustment work is undesirable. This is the case e.g. in pipetting devices in modern automatic analyzers. In these, a relatively large number of primary vessels have to be disposed in a relatively small space, and consequently the pipetting needle has to be exactly positioned relative to the individual primary or secondary vessels to ensure reliable pipetting. Exact positioning should also be obtained with minimum labor.
International Patent Application Publication No. WO 91/16675 describes an automatic pipetting device, in which the vessel carrier units each carry an arrangement of vessels, each vessel carder unit being disposed on a predetermined defined position on a working surface, and wherein a single reference member is provided on a given point of the working surface and is permanently and directly connected to the working surface and serves for adjustment of the needle-conveying system. In this known pipetting device, the pipetting needle is connected to a sensor circuit. For adjustment of the needle-conveying system, the needle is used to detect the position of the walls of the single reference member. An important disadvantage of this known device is that the adjustment accuracy that can be achieved therewith is limited by mechanical tolerances between the single reference member and the individual pipetting positions, as well as by the fact that any somewhat inaccurate positioning of the vessel carrier units or deformation of the pipetting needle are disregarded for the adjustment. The consequences of inaccurate adjustment are catastrophic in a system which relies on very accurate adjustment. A pipetting needle incorrectly guided for this reason not only misses its targets, but also damages other system components such as reaction vessels and reagent containers. In addition, the pipetting needle itself may be damaged (deformed) and even broken.